This invention pertains to high plasticization-resistant chemically cross-linked polymeric membranes such as cross-linked cellulose acetate (CA) membranes and methods of making the same. This invention also pertains to applications of these cross-linked polymeric membranes not only for a variety of gas separations such as separations of carbon dioxide/methane, hydrogen/methane, oxygen/nitrogen, carbon dioxide/nitrogen, olefin/paraffin, iso/normal paraffins, polar molecules such as water, hydrogen sulfide and ammonia/mixtures with methane, nitrogen, or hydrogen and other light gases separations, but also for liquid separations such as pervaporation and desalination.
Membrane-based technologies have advantages of both low capital cost and high-energy efficiency compared to conventional separation methods. Polymeric membranes have proven to operate successfully in some industrial gas separations such as separation of N2 from air and separation of CO2 from natural gas. Cellulose acetate (CA) is one of a few polymers currently being used in commercial gas separations. For example, UOP's Separex™ CA membrane is extensively used for CO2 removal from natural gas. Nevertheless, CA membranes are not without problems. They are limited in a number of properties including selectivity, permeability, chemical and thermal stability. For example, natural gas often contains substantial amounts of heavy hydrocarbons and water, either as entrained liquid, or in vapor form, which may lead to condensation within the membrane modules. The gas separation properties of CA membranes are damaged by contact with liquid hydrocarbons or/and liquid water. The presence of more than modest ppm levels of hydrogen sulfide, especially in conjunction with water and heavy hydrocarbons, is also potentially damaging. Therefore, precautions must be taken to remove the entrained liquid water and heavy hydrocarbons upstream of the membrane separation steps. Another issue of CA polymer membranes that still needs to be addressed for gas separations is the plasticization of CA polymer by condensable gases such as CO2 and propylene (C3H6) that will therefore lead to swelling and significant increase in the permeabilities of all components in the feed and a decrease in the selectivity of CA membranes. For example, the permeation behavior of CO2 in CA is unusual, compared to some other glassy polymers, in that after a certain pressure, the permeability coefficient begins to increase with pressure due to the onset of plasticization by the CO2. The high concentration of sorbed CO2 penetrant leads to increased segmental motions, and, consequently, the transport rate of the penetrant is also enhanced. The challenge of treating gas that contains relatively large amounts of CO2, such as more than about 10%, is still particularly difficult. See Sada et al., J. POLYM. SCI. B: POLYM. PHYS., 26: 1035 (1988); Sada et al., J. POLYM. SCI. B: POLYM. PHYS., 28: 113 (1990); Donohue, et al., J. MEMBR. SCI., 42: 197 (1989).
Some new high-performance polymers such as polyimides (PIs), poly(trimethylsilylpropyne) (PTMSP), and polytriazole exhibit a high ideal selectivity for CO2 over CH4 when measured with pure gases at modest pressures in the laboratory. However, the selectivity obtained under mixed gas, high pressure conditions is much lower. In addition, gas separation processes based on glassy solution-diffusion membranes frequently suffer from plasticization of the stiff polymer matrix by the sorbed penetrant molecules such as CO2 or C3H6. Plasticization of the polymer represented by membrane structure swelling and significant increase in the permeabilities of all components in the feed occurs above the plasticization pressure when the feed gas mixture contains condensable gases.
Thus, there is still a critical need for new high-performance membranes that will provide and maintain adequate performance under conditions of exposure to organic vapors, high concentrations of acid gases such as CO2 and hydrogen sulfide, and water vapor that are commonplace in natural gas treatment.
Conventional methods for stabilizing polymeric membranes are either annealing or cross-linking. Cross-linking is a useful method to suppress polymer membrane plasticization. Polymer membrane cross-linking methods include thermal treatment, radiation, chemical cross-linking, UV-photochemical, etc. Cross-linking offers the potential to improve the mechanical and thermal properties of a membrane. Cross-linking can be used to increase membrane stability in the presence of aggressive feed gases and to simultaneously reduce plasticization of the membrane. Normally, cross-linked polymer membranes have a high resistance to plasticization. See Koros, et al., US 20030221559 (2003); Jorgensen, et al., US 2004261616 (2004); Wind, et al., Macromolecules, 36: 1882 (2003); Patel, et al., ADV. FUNC. MATER., 14 (7): 699 (2004); Patel, et al., MACROMOL. CHEM. PHY., 205: 2409 (2004).
In this invention, we disclose a chemical cross-linking method for the preparation of high plasticization-resistant chemically cross-linked polymeric membranes, and applications using the same. One goal of this invention is to reduce undesirable effects caused by condensable gases such as CO2 and propylene (C3H6) induced plasticization (swelling) of polymeric membranes for gas separations. The polymer structure was stabilized by the incorporation of cross-linking agents and the formation of covalently interpolymer-chain-connected rigid networks.